The flux-cored wires used for gas-shielded welding usually consist of a tubular outer metal sheath, also called a foil, and of a central core comprising filling elements, for example a metal powder.
Such flux-cored wires are used in many gas-shielded welding procedures, in particular in the welding of normalized structural steel, TMCP (Thermo Mechanical Control Process) steel and tempered-and-annealed steel.
At the present time there are three types of flux-cored wire which can be used for the gas-shielded welding of metal workpieces, especially of structural steel workpieces, namely "basic", "rutile" and "slag-free" wires, also called "metal-cored" wires. These various flux-cored wires are differentiated by the nature of the various elements of which they are composed.
Thus, depending on the type of flux-cored wire used in the welding process, there is a greater or lesser tendency for the flux-cored wire to undergo soft melting, arc stability may increase or decrease, positional welding may become easier or harder, the amount of molten metal spattered may increase or decrease, etc.
Likewise, depending on the type of flux-cored wire used in the welding process, and therefore depending on the nature and content of the various elements of which this flux-cored wire is composed, the metallurgical properties of a weld obtained, i.e. of a metal deposited, will vary greatly.
Thus, it is known that the characteristics of the weld depend on the content of the metal deposited in terms of its various elements such as, in particular, the elements: oxygen, titanium, niobium, vanadium, hydrogen, etc.
By way of example, Table I below indicates, on the one hand, the main elements involved in the constitution of the 3 aforementioned different types of flux-cored wires and, on the other hand, for each of them, the operational and compositional consequences which conventionally result therefrom.
TABLE I ______________________________________ TYPE OF FLUX- CORED WIRE RUTILE BASIC SLAG-FREE ______________________________________ Non-metallic TiO.sub.2, SiO.sub.2, CaF.sub.2, CaCO.sub.3, &lt;1% Ionizing elements in the Al.sub.2 O.sub.3, Ionizing MgO, Ionizing elements wire elements elements Metallic elements Deoxidizing Deoxidizing Deoxidizing in the wire agents, alloying agents, alloying agents, alloy- elements elements ing elements Operating con- Soft melting Globular transfer Soft melting sequences Stable arc Splatter Stable arc Positional weld- Positional weld- ing easy ing difficult Wide range of metal transfer by spraying Content of the O.sub.2 : 600-1000 O.sub.2 : 300-400 ppm O.sub.2 : 600-1000 deposited metal ppm Ti: adjustable ppm (weld) Ti: 300-800 ppm Nb: adjustable Ti: adjustable Nb: 100-300 ppm V: adjustable Nb: adjustable V: 50-250 ppm H.sub.2 : &lt;5 ml V: adjustable H.sub.2 : 4 to 15 ml H.sub.2 : 1 to 6 ml ______________________________________
It is apparent from the above Table I that the flux-cored wires of the "rutile" type, based on titanium oxide (TiO.sub.2), have better operating properties than the other types of wire: very good arc stability, a regime of metal transfer by axial spraying for a wide range of parameters resulting in the almost complete absence of spatter during welding, slag with a high melting point allowing a high deposition rate in positional welding, etc.
However, from the composition of its slag, i.e. of the filling elements of which its central core is composed, the "rutile"-type flux-cored wire has the drawback of leading to a weld containing residual elements (oxygen, niobium, vanadium, etc.) which tend to prevent optimum mechanical properties of the deposited metal, i.e. the weld, being achieved.
More specifically, the oxygen content of the deposited metal, although this may vary depending on the nature and quantity of deoxidizing elements present in the filling powder, cannot, in the case of "rutile"-type wires, be reduced to as low a level as, for example, in the case of "basic"-type wires.
It follows that a weld produced using a "rutile"-type wire has a lower ductile fracture energy due to a higher amount of inclusion.
Likewise, since the slag of the "rutile"-type wire is mostly composed of rutile, i.e. of titanium oxide (TiO.sub.2), titanium is inevitably found in the deposited metal and in a quantity which varies depending, in particular, on the oxido-reduction reactions occurring in the arc and on the metal-slag exchanges taking place within the weld pool, i.e. within the molten metal.
The titanium content of the deposited metal cannot therefore be adjusted as required, but depends closely on all the chemical elements present, which must be balanced depending on the mechanical properties (tensile strength, yield stress, etc.) which the weld must have, which themselves depend on the type of steel that has to be welded.
Furthermore, with regard to the niobium and vanadium contents of the deposited metal, these also cannot be reduced below a certain threshold, given that these elements exist as impurities in the natural titanium oxides normally used for manufacturing welding products.
The use of synthetic and therefore relatively pure, titanium oxide partly solves this problem, but to the detriment of a significant increase in the cost of the flux-cored wire and at the cost of greater complexity of the process for manufacturing the latter.
Extensive research carried out with the aim of improving the metallurgical properties of "rutile"-type flux-cored wires have resulted in flux-cored wires exhibiting a "titanium-boron" effect.
Specifically, it has been shown that the presence of a very small quantity of boron, in general 20 to 60 ppm, in the deposited metal considerably retards the appearance of proeutectoid ferrite at the austenitic grain boundaries as the weld deposit cools down.
Thus, transformation of the austenite can occur by intragranular nucleation of ferrite on the fine inclusions, which are always present in a weld, provided that the latter contain a certain quantity of titanium.
A structure having very fine ferrite grains is usually called "acicular ferrite"; it has improved tensile properties (tensile strength, yield stress, etc.) and toughness (Charpy V-notched impact strength, CTOD, etc.).
In other words, during cooling of the weld, the titanium-boron effect prevents the formation of coarse proeutectoid ferrite, which is very prejudicial to the toughness of the weld, and finally produces a structure of the acicular-ferrite type, consequently making the welds produced using wires of the rutile type (but also of the slag-free type) compatible, in the as-welded state, with the most stringent industrial requirements, which could not be met by wires of the basic type.
However, although the results obtained are satisfactory in the as-welded state, the same does not apply when the weld undergoes a postwelding heat treatment necessary, in particular, for stress relieving the weldment, especially in the case of joining very thick workpieces.
This is because it has been observed that a weld produced by means of a "titanium-boron" rutile wire, which was subjected to a subsequent heat treatment, lost its metallurgical properties and therefore the benefit provided by the "titanium-boron" effect.
Consequently, several publications report tests relating to the formulation of filling powders for flux-cored wires, in particular of "rutile" type, which were intended to improve the behaviour of the welds after a postwelding heat treatment.
Furthermore, among the various elements which may be found in flux-cored wires, mention should be made of nitrogen.
Thus, document JP-A-63,220,996 teaches a flux-cored wire whose sheath contains 220 ppm nitrogen.
Furthermore, documents JP-A-63,278,697 describes a flux-cored wire containing 1-12% metal fluoride, 0.1-1.5% metal carbonate, 0.2-3% of one or more mineral silicates and silica chlorides, 0.2-2% silica, 1-2.5% manganese, 0.05-0.3% titanium and 0.1-1% magnesium; the weight of the filling flux representing from 10 to 30% of the total weight of the flux-cored wire.
Moreover, the document I.S.I.J. INTERNATIONAL, Vol. 35, 1995, VO 10 teaches, moreover, that the low-temperature impact strength of the weld is improved by increasing the nitrogen content up to levels reaching the preferred values of about 80 ppm.
However, at the present time none of the existing flux-cored wires, in particular of the rutile type, produces a weld having good toughness and impact-strength properties after a postwelding heat treatment.